Subscriber line interface circuits (SLICs) are often present in a central office exchange of a telecommunications network or remote locations thereto for use in providing a communication interface between a digital switching network of a central office and an analog subscriber line. The analog subscriber line connects to a subscriber station or telephone instrument at a location that is remote from the central office exchange.
The analog subscriber line and subscriber equipment (e.g., a telephone) form a subscriber loop. The interface requirements of a SLIC typically require high voltages and currents for control signaling with respect to the subscriber equipment on the subscriber loop. Voiceband communications are typically low voltage analog signals on the subscriber loop. Accordingly, the SLIC performs various functions with respect to voiceband and control signaling between the subscriber equipment and the central exchange.
SLIC functionality has generally been implemented in multiple integrated circuits (ICs), or combinations of ICs and discrete elements. Typically, significant high voltage circuitry is included in one IC to provide various high voltage functionality of a SLIC. Accompanying low voltage IC's are used to perform control functions for the high voltage portion and also to perform low voltage tasks, voice signal processing, and to provide an interface to system circuitry, e.g., a system on a chip (SOC) such as a digital signal processor (DSP) or other digital processing circuit of a central office or similar location. In turn, the DSP is coupled to provide system input/output (I/O) signals to other locations in the telecommunications network. In other implementations, instead of a DSP interface, the SLIC may couple directly into a switching system.
Typically, a significant number of wires or signal lines are used to connect low voltage portions of a SLIC with the high voltage portion. Furthermore, different SOCs or DSPs used in a system can require different information from a SLIC. That is, different DSPs have different capabilities with respect to signal processing. Some DSPs include capabilities for analog signal processing such as codec functionality and filtering, while other DSPs strictly handle digital signal processing for system requirements such as code compression, call processing, echo cancellation, among others. Accordingly, different SLIC configurations are needed to interface with different DSPs.
These different SLIC configurations typically require completely different designs, often in different process technologies. Such different designs are not readily reused across different process technologies and different SLIC configurations.
Another limitation with respect to SLIC design is that because of the criticalities of the different low voltage and high voltage components, it is typically difficult to port a given design across different process technologies. Thus, a SLIC design implemented in one process technology is not easily ported to another technology, owing to differences in device characteristics. This typically requires the need for significant calibration, trimming and other design-intensive matching of devices.
The high voltage portion of a SLIC typically includes bidirectional amplifiers (either current or voltage mode). These traditional amplifiers are high-voltage operational amplifiers that provide for precise bidirectional current or voltage gain applications. These bidirectional amplifiers require large output transistors to source and sink the output current, and at relatively high currents these output transistors consume significant real estate. Furthermore, the amplifier must operate over a full power supply range (i.e., positive and negative supplies), requiring many high voltage transistors and careful design.
A bidirectional current amplifier can be formed using current mirrors, which are a key design element used in analog circuit design and especially in IC analog design. Current mirrors allow an input current to be replicated. Current mirrors can have an arbitrary gain (including unity) and multiple outputs. Current mirrors can be implemented in variety of ways using different types of transistors, resistors and operational amplifiers. The most common current mirrors use the fact that IC transistors have good matching and can be used to build simple current mirrors. Precision current mirrors such as those used in a high voltage operational amplifier usually include circuitry that increases the input voltage drop required for operation. However, in low voltage designs, this can be a problem.
A need thus exists for improved manners of implementing subscriber line interface circuitry.